The present invention relates to liquid crystalline compounds and liquid crystal compositions. More specifically, the invention relates to liquid crystalline ester compounds preferable as component of liquid crystal compositions particularly for TN mode, STN mode, or TFT mode, to liquid crystal compositions comprising the compounds, and to liquid crystal display devices fabricated by using the liquid crystal composition as a material to be incorporated therein. In this specification, the term xe2x80x9cliquid crystalline compoundsxe2x80x9d is used as a generic name both for the compounds which exhibit a liquid crystal phase and the compounds which do not exhibit a liquid crystal phase but are useful as component of liquid crystal compositions.
Liquid crystal display devices employ optical anisotropy and dielectric anisotropy of liquid crystal substances; the devices are classified by their displaying method into various modes such as twisted nematic (TN) mode, dynamic scattering (DS) mode, guest-host (GH) mode, deformation of aligned phases (DAP) mode, super-twisted nematic (STN) mode, electrically controlled birefringence (VCB, ECB, or TB) mode, and vertical alignment (VA) mode; and properties of liquid crystal substances suitable for each of the modes are different. It is necessary to the liquid crystal substances used for any mode of display devices that the liquid crystal substances are stable against moisture, air, heat, and light. For liquid crystal materials employed in display devices particularly using active matrix driving, especially high voltage holding ratio is required.
In recent years, investigations for lowering driving voltage of liquid crystal display devices have been conducted and thus liquid crystal materials having large absolute value of dielectric anisotropy xcex94xcex5 have been sought. For instance, in TN mode, liquid crystal materials having positive and large xcex94xcex5 value make driving of the devices at a low voltage possible, and in VA mode, liquid crystal materials having negative and large xcex94xcex5 make the low voltage driving possible. Besides, in the case of in-plane switching (IPS) mode employing the change in birefringence of liquid crystal materials by on-off switching of transverse electric field (electric field directed parallel to the plane of the substrates), liquid crystal materials having large xcex94xcex5 or xe2x88x92xcex94xcex5 have been sought.
In order to increase response speed of liquid crystal display devices, low viscosity is required of liquid crystal materials. In order to make display in a practically wide temperature range possible, excellent miscibility at low temperatures and high clearing point are required of liquid crystal materials.
Further, it is also required that liquid crystal materials used for STN mode have steep threshold characteristics.
Since most favorable optical anisotropy value xcex94n of liquid crystal compositions is different depending on display mode and cell thickness, xcex94n value required of liquid crystal compounds is in a wide range.
It is known that when the benzene ring at the core part of liquid crystal compounds is replaced by cyclohexane ring, xcex94n value of the liquid crystal compounds becomes small (KIKAN KAGAKU SOSETSU (Seasonal publication, General Remarks in Chemistry) No. 22, 1994, Chemistry of Liquid Crystals, page 43). In connection with the structure of the core part, knowledge in the past on the characteristics of liquid crystalline compounds classified by the presence or absence of an aromatic ring such as benzene ring and pyrimidine ring at the core part is described below.
Liquid crystalline compounds having no aromatic ring at the core part generally have small xcex94n value. For instance, 4-(4-pentylcyclohexyl)cyclohexanecarbonitrile (Compound (10)) is described on page 44 of the Chemistry of Liquid Crystals described above. Extrapolated xcex94n value of this compound is 0.06 (according to the determination by the present inventors, the value is 0.068). 
As shown in Example 31 (Comparative Example 1) described below, extrapolated xcex94xcex5 value of the compound (10) is as small as 3.7 and extrapolated value of viscosity is as high as 55.0 mPaxc2x7s according to the determinations by the present inventors. Thus, whereas the compound (10) has small xcex94n value, it is difficult to actualize low voltage driving of liquid crystal display devices due to its small xcex94xcex5 value, and it is also difficult to actualize high speed response due to its high viscosity. Besides, its voltage holding ratio is low, and therefore the compound cannot be used for liquid crystal display devices for TFT mode.
In Mol. Cryst. Liq. Cryst., 1991, Vol. 204, pages 86 and 84, 4-(4-propylcyclohexyl)cyclohexyl trifluoromethyl ketone (Compound (11)) and 4-(4-propylcyclohexyl)cyclohexanecarboxylic acid 2,2,2-trifluoroethyl ester (Compound (12)) are described, respectively. Whereas these compounds also have small xcex94n value, their xcex94xcex5 value are as small as 5.1 and 2.0, respectively, and thus it is difficult to actualize low voltage driving. 
At present, exploitation of compounds having large absolute value of xcex94xcex5 and low viscosity while having small xcex94n value are required.
As the compounds having an aromatic ring at the core part and having large xcex94xcex5 value, following compounds in which cyano group or halogen atoms are bonded to the benzene ring at a terminal of molecule are known. 
Whereas all these compounds have medium or large xcex94n value since they have an aromatic ring, the compound (13) has low voltage holding ratio and thus can not stand in the use for TFT. Whereas Compounds (14) and (15) have good voltage holding ratio, they have disadvantages that clearing point is low and viscosity is high. As shown in Example 32 (Comparative Example 2) described below, clearing point of the compound (14) is as low as 95.7xc2x0 C. and its viscosity is as high as 46.3 mPaxc2x7s. Accordingly, exploitation of compounds having high clearing point, low viscosity, and high voltage holding ratio while having medium or large xcex94n value are currently required.
An object of the present invention is to solve the defects in conventional technology. Another object of them present invention is to provide liquid crystalline compounds having large xcex94xcex5 value and low viscosity while having small xcex94n value, or liquid crystalline compounds having high voltage holding ratio, high clearing point, and low viscosity while having medium or large xcex94n value; to provide liquid crystal compositions comprising one side or both sides of the compounds described above thereby low voltage driving, high speed response, and driving at a wide temperature range of liquid crystal display devices become possible; and to provide liquid crystal display devices fabricated by using the liquid crystal composition as material to be incorporated in the devices.
As a result of the investigations by the present inventors to achieve the objects described above, it has been found that compounds having an ester group in which a halogen atom or halogenated alkyl group is bonded to the carbon atom of carbonyl, and having no aromatic ring at the core part have small xcex94n value, large xcex94xcex5 value, low viscosity, and high voltage holding ratio, that such compounds as described above but having an aromatic ring at the core part have medium or large xcex94n value, high voltage holding ratio, high clearing point, and low viscosity, and that liquid crystal compositions comprising the former compound or the latter compound are the most suitable materials for low voltage driving, driving at a wide temperature range to be used, and high speed response of diversified liquid crystal display devices, leading to the accomplishment of the present invention.
That is, the present invention is summarized as follows:
(1) An ester compound expressed by the general formula (1) 
xe2x80x83wherein R1 represents hydrogen atom, cyano group, nitro group, amino group, a halogen atom, a straight chain or branched alkyl group having 1 to 20 carbon atoms, or a straight chain or branched alkenyl group having 2 to 20 carbon atoms; one or more hydrogen atoms in these alkyl group and alkenyl group may be replaced by halogen atoms; one or more xe2x80x94CH2xe2x80x94 in these alkyl group and alkenyl group may be replaced by oxygen atoms or sulfur atoms, but there is no case where oxygen atoms are adjacent; and one or more carbon atoms in these alkyl group and alkenyl group may be replaced by silicon atoms;
rings A1 to A5 each independently represent 1,4-cyclo-hexylene, 1,4-phenylene, 1-cyclohexene-1,4-diyl, 3-cyclohexene-1,4-diyl, 1-sila-1,4-cyclohexylene, or 4-sila-1,4-cyclohexylene; one or more xe2x80x94CH2xe2x80x94 in the 1,4-cyclohexylene, 1-cyclohexene-1,4-diyl, and 3-cyclohexene-1,4-diyl may each independently be replaced by oxygen atoms, sulfur atoms, xe2x80x94COxe2x80x94, xe2x80x94CSxe2x80x94, SiH2xe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94CF2xe2x80x94, or xe2x80x94CFHxe2x80x94, but there is no case where oxygen atoms are adjacent; hydrogen atoms which bond to carbon atoms at position 2 and position 3, respectively, in the 1-cyclohexene-1,4-diyl or 3-cyclohexene-1,4-diyl may each independently be replaced by fluorine atoms; one or more hydrogen atoms in the 1,4-phenylene may each independently be replaced by halogen atoms, halogenated alkyl groups having 1 to 3 carbon atoms, methyl groups, cyano groups, or nitro groups; and one or more xe2x80x94CHxe2x95x90 is the 1,4-phenylene may each independently be replaced by nitro groups;
bonding groups Za to Zd each independently represent single bond, xe2x80x94CH2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, or an alkylene group or alkenylene group having 2 to 4 carbon atoms; one or more hydrogen atoms in these groups may be replaced by halogen atoms; and xe2x80x94CH2xe2x80x94 in these groups may each independently be replaced by oxygen atom, sulfur atom, xe2x80x94COxe2x80x94, xe2x80x94CSxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94CF2xe2x80x94, or xe2x80x94CFHxe2x80x94, but there is no case where oxygen atoms are adjacent;
bonding group Ze represents single bond, an alkylene group having 1 to 10 carbon atoms, or alkenylene group; and xe2x80x94CH2xe2x80x94 in these groups may each independently be replaced by oxygen atom, sulfur atom, xe2x80x94COxe2x80x94, xe2x80x94CF2xe2x80x94, xe2x80x94CFHxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CFxe2x95x90CHxe2x80x94, or xe2x80x94Cxe2x89xa1Cxe2x80x94, but there is no case where oxygen atoms are adjacent;
n1 to n3 are each independently 0 or 1, but n1xe2x89xa6n2xe2x89xa6n3;
Y1 and Y2 each independently represent oxygen atom or sulfur atom;
R2 represents a halogen atom, a straight chain or branched alkyl group having 1 to 20 carbon atoms, or straight chain or branched alkenyl group having 2 to 20 carbon atoms, one or more hydrogen atoms in these alkyl group and alkenyl group are replaced by halogen atoms; one or more xe2x80x94CH2xe2x80x94 in these groups may be replaced by oxygen atoms or sulfur atoms, but there is no case where oxygen atoms are adjacent; and one or more carbon atoms in these groups may be replaced by silicon atoms;
provided that when A5 represents unsubstituted 1,4-phenylene in this compound, then Zd is single bond, Ze is single bond or an alkylene group having 1 to 10 carbon atoms in which alkylene group xe2x80x94CH2xe2x80x94 may independently be replaced by oxygen atom, sulfur atom, xe2x80x94CF2xe2x80x94, xe2x80x94CFHxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94CFxe2x95x90CFxe2x80x94, xe2x80x94CFxe2x95x90CHxe2x80x94, or xe2x80x94Cxe2x89xa1Cxe2x80x94, but there is no case where oxygen atoms are adjacent, and A4 is not 1,4-phenylene, and that when A5 represents cyclohexylene ring and Ze represents single bond, then there is no case where R2 represents an optically active monofluoroalkyl group, optically active monofluoroalkenyl group, or one of these groups in which one xe2x80x94CH2xe2x80x94 is replaced by oxygen atom; and
each atom which constitutes this compound may be replaced by its isotope.
(2) The ester compound recited in (1) above wherein ring A5 is 1,4-cyclohexylene, 1-cyclohexene-1,4-diyl, or 3-cyclohexene-1,4-diyl; one or more xe2x80x94CH2xe2x80x94 in these groups may each independently be replaced by oxygen atoms, sulfur atoms, xe2x80x94COxe2x80x94, xe2x80x94CSxe2x80x94, xe2x80x94SiH2xe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94CF2xe2x80x94, or xe2x80x94CFHxe2x80x94, but there is no case where oxygen atoms are adjacent; hydrogen atoms which bond to carbon atoms at position 2 and position 3, respectively, in the 1-cyclohexene-1,4-diyl or 3-cyclohexene-1,4-diyl may be replaced by fluorine atoms; and both Y1 and Y2 are oxygen atoms in the general formula (1).
(3) The ester compound recited in (2) above wherein Ze is single bond in the general formula (1).
(4) The ester compound recited in (3) above wherein R2 is a straight chain alkyl group having 1 to 5 carbon atoms in which alkyl group two or more hydrogen atoms are replaced by fluorine atoms in the general formula (1).
(5) The ester compound recited in (4) above wherein both n1 and n2 are 0, n3 is 1, both rings A4 and A5 are 1,4-cyclohexylene in which one or more xe2x80x94CH2xe2x80x94 may each independently be replaced by oxygen atoms, but there is no case where oxygen atoms are adjacent; and Zd is single bond in the general formula (1).
(6) The ester compound recited in (1) above wherein both n1 and n2 are 0, n3 is 0, and at least one of rings A3, A4, and A5 is 1-cyclohexene-1,4-diyl or 3-cyclohexene-1,4-diyl in the general formula (1).
(7) The ester compound recited in (1) above wherein ring A5 is 1,4-phenylene in which phenylene group one or more hydrogen atoms may each independently be replaced by halogen atoms, halogenated alkyl groups having 1 to 3 carbon atoms, methyl or nitro groups, and one or more xe2x80x94CHxe2x95x90 may each independently be replaced by nitrogen atoms in the general formula (1).
(8) The ester compound recited in (7) above wherein ring A5 is. 1,4-phenylene in which one or more hydrogen atoms are replaced by fluorine atoms in the general formula (1).
(9) The ester compound recited in (1) above 1 wherein R1 is a straight chain alkyl group having 1 to 20 carbon atoms or a straight chain alkenyl group having 2 to 20 carbon atoms; one or more hydrogen atoms in which groups may be replaced by halogen atoms, one or more xe2x80x94CH2xe2x80x94 in the groups may be replaced by oxygen atoms or sulfur atoms, but there is no case where oxygen atoms are adjacent; and one or more carbon atoms in the groups may be replaced by silicon atoms in the general formula (1).
(10) A liquid crystal composition comprising at least one ester compound recited in any one of (1) to (9) above.
(11) A liquid crystal composition comprising, as a first component, at least one ester compound recited in any one of (1) to (9) above, and comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (2), (3), and (4) 
xe2x80x83wherein R3, Y3, L1, L2, Z1, and Z2 may be the same or different among each of the formulas;
R3 represents an alkyl group having 1 to 10 carbon atoms in which alkyl group one or not-adjacent two or more methylene groups may be replaced by oxygen atoms or xe2x80x94CHxe2x95x90CHxe2x80x94; and any hydrogen atom in the alkyl group may be replaced by fluorine atom;
Y3 represents fluorine atom, chlorine atom, xe2x80x94OCF3, xe2x80x94OCF2H, xe2x80x94CF3, xe2x80x94CF2H, xe2x80x94CFH2, xe2x80x94OCF2CF2H, or xe2x80x94OCF2CFHCF3;
L1 and L2 each independently represent hydrogen atom or fluorine atom;
Z1 and Z2 each independently represent xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or single bond;
ring B represents trans-1,4-cyclohexylene or 1,3-dioxane-2,5-diyl, or 1,4-phenylene in which hydrogen atom may be replaced by fluorine atom;
ring C represents trans-1,4-cyclohexylene, or 1,4-phenylene in which hydrogen atom may be replaced by fluorine atom; and
each atom which constitutes these compounds may be replaced by its isotope.
(12) A liquid crystal composition comprising, as a first component, at least one compound recited in any one of (1) to (9) above, and comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by the general formula (5) or (6) 
xe2x80x83wherein R4 and R5 each independently represent an alkyl group having 1 to 10 carbon atoms in which alkyl group one or not-adjacent two or more methylene groups may be replaced by oxygen atoms or xe2x80x94CHxe2x95x90CHxe2x80x94; and hydrogen atom in the group may be replaced by fluorine atom;
Y4 represents xe2x80x94CN or xe2x80x94Cxe2x89xa1Cxe2x80x94CN;
ring E represents trans-1,4-cyclohexylene, 1,4-phenylene, 1,3-dioxane-2,5-diyl, or pyrimidine-2,5-diyl;
ring G represents trans-1,4-cyclohexylene or pyrimidine-2,5-diyl, or 1,4-phenylene in which hydrogen atom may be replaced by fluorine atom;
ring H represents trans-1,4-cyclohexylene or 1,4-phenylene;
Z3 represents xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, or single bond;
L3, L4, and L5 each independently represent hydrogen atom or fluorine atom;
b, c, and d are each independently 0 or 1; and
each atom which constitutes these compounds may be replaced by its isotope.
(13) A liquid crystal composition comprising, as a first component, at least one compound recited in any one of (1) to (9) above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by any of the general formula (2), (3), and (4) described above, and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed any one of the general formulas (7), (8), and (9) 
xe2x80x83wherein R6, R7, I, J, and K may be the same or different among each of the formulas;
R6 and R7 each independently represent an alkyl group having 1 to 10 carbon atoms in which alkyl group one or not-adjacent two or more methylene groups may be replaced by oxygen atoms or xe2x80x94CHxe2x95x90CHxe2x80x94; and any hydrogen atom in the group may be replaced by fluorine atom;
ring I, ring J, and ring K each independently represent trans-1,4-cyclohexylene or pyrimidine-2,5-diyl, or 1,4-phenylene in which hydrogen atom may be replaced by fluorine atom;
Z4 and Z5 each independently represent xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or single bond; and
each atom which constitutes these compounds may be replaced by its isotope.
(14) A liquid crystal composition comprising, as a first component, at least one compound recited in any one of (1) to (9) above, comprising, as a second component, at least one compound selected from the group consisting of the compounds expressed by the general formula (5) or (6) described above, and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (7), (8), and (9) described above.
(15) A liquid crystal composition comprising, as a first component, at least one compound recited in any one of (1) to (9) above, comprising, as a part of a second component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (2), (3), and (4), comprising, as another part of the second component, at least one compound selected from the group consisting of the compounds expressed by the general formula (5) or (6) described above, and comprising, as a third component, at least one compound selected from the group consisting of the compounds expressed by any one of the general formulas (7), (8), and (9) described above.
(16) A liquid crystal composition recited in any one of (10) to (15) above in which at least one optically active compound is further comprised.
(17) A liquid crystal display device comprising a liquid crystal composition recited in any one of (10) to (16) above.
Ester compounds of the present invention expressed by the general formula (1) have two to five rings at its core part. Also, in the compounds of the present invention, a halogen atom, halogenated alkyl group, or halogenated alkenyl group is bonded to the carbon atom of carbonyl in at least one ester group. Since the compounds of the present invention have such a specific structure as described above, the compounds which have no aromatic ring at the core part have small xcex94n, large xcex94xcex5, low viscosity, and high voltage holding ratio; and the compounds having aromatic ring at the core part have medium or large xcex94n, large xcex94xcex5, high clearing point, low viscosity, and high voltage holding ratio.
Such effect was achieved for the first time by the fact that the liquid crystalline compounds have a halogen atom or halogenated alkyl group bonding to the carbon atom of carbonyl in an ester group in the molecule.
By using compounds having no aromatic ring at the core part among liquid crystalline compounds expressed by the general formula (1), liquid crystal compositions which make low voltage driving and high speed response of liquid crystal display devices possible and have small xcex94n can be produced. By using compounds having an aromatic ring at the core part, on the other hand, liquid crystal compositions which make driving at a wide temperature range and high speed response possible, and have medium or large xcex94n can be produced.
Any of the groups shown as R1 makes the compounds expressed by the general formula (1) develop excellent characteristics. The compounds in which R1 is an alkyl group having 1 to 7 carbon atoms, alkenyl group having 2 to 7 carbon atoms, or alkenyl group wherein two fluorine atoms are directly bonded to double bond develop low viscosity.
Among those, the compounds in which R1 is an alkyl group can stand active matrix driving since they develop high voltage holding ratio, and the compounds in which R1 is an alkenyl group improve particularly steepness of threshold characteristics.
The compounds in which R1 is a fluoroalkyl group having 1 to 7 carbon atoms are excellent in miscibility at low temperatures, and the compounds in which R1 is an alkyl group wherein one CH2 is replaced by oxygen atom, particularly alkoxymethyl group increase xcex94xcex5 value. Any of these compounds in which R1 is fluoroalkyl group or alkoxyalkyl group have high voltage holding ratio.
Any group shown as bonding groups Za to Zd in the general formula described above make the compounds which are expressed by the general formula (1) develop excellent characteristics. The compounds in which bonding groups Za to Zd are single bonds develop high clearing point, low viscosity, excellent miscibility with other liquid crystalline compounds or liquid crystal compositions at low temperatures, and high voltage holding ratio. Whereas xe2x80x94CH2CH2xe2x80x94 is slightly inferior in the aspect of viscosity compared with single bond, it makes the compounds develop more excellent miscibility at low temperatures. Whereas xe2x80x94COOxe2x80x94 is slightly inferior in voltage holding ratio compared with single bond, it makes the compounds develop high clearing point and large xcex94xcex5 value. Besides, xe2x80x94CF2Oxe2x80x94 makes the compounds develop low viscosity and comparatively large xcex94xcex5 compared with single bond. Whereas xe2x80x94CHxe2x95x90CHxe2x80x94 is slightly inferior in voltage holding ratio compared with single bond, it makes the compounds develop low viscosity and steep threshold characteristics.
Any group shown as bonding group Ze in the general formula (1) makes the compounds develop excellent characteristics. Particularly, when Ze is single bond, it makes the compounds develop high clearing point, low viscosity, excellent miscibility with other liquid crystalline compounds or liquid crystal compositions at low temperatures, and high voltage holding ratio, and when Ze is xe2x80x94CH2CH2xe2x80x94, it makes the compounds develop excellent miscibility at low temperatures whereas it is slightly inferior in the aspect of viscosity compared with single bond.
In the general formula (1), n1 to n3 are each independently 0 or 1 provided that n1xe2x89xa6n2xe2x89xa6n3. As specific combination of n1, n2, and n3, while four kind of combinations of (n1, n2, n3)=(0, 0, 0), (0, 0, 1), (0, 1, 1), or (1, 1, 1) can be mentioned, the compounds of (n1, n2, n3)=(0, 0, 0) have low viscosity and are excellent in miscibility at low temperatures, the compounds of (n1, n2, n3)=(0, 0, 1) have comparatively high clearing point and comparatively low viscosity, and the compounds of (n1, n2, n3)=(0, 1, 1) have high clearing point.
While any terminal substituent R2 in the general formula (1) described above makes the compounds develop excellent characteristics, particularly CF3 and CF2CF3 make them develop large xcex94xcex5 and low viscosity, and CF2H makes the compounds develop high clearing point whereas it is slightly inferior in xcex94xcex5 compared with CF3.
While various type of structures can be applied at the core part in the general formula (1) described above, any of them make the compounds develop excellent characteristics. Among them, some examples are shown in following general formulas (1-1) to (1-32): 
Among the compounds described above, the compounds expressed by one of the general formulas (1-1) to (1-8) have small xcex94n. The compounds in which the core part is formed only cyclohexane rings like the compounds of the general formula (1-1) or (1-4) have low viscosity, and the compounds having dioxane ring at the core part like the compounds of the general formulas (1-2), (1-5), or (1-8) have large xcex94xcex5. Further, the compounds having cyclohexene ring like those of the general formulas (1-3), (1-6), (1-7), or (1-8) are excellent in miscibility at low temperatures.
Among the compounds described above, the compounds expressed by one of the general formulas (1-9) to (1-20) have medium extent of xcex94n, and xcex94xcex5 value of the compounds becomes larger as the number of fluorine atoms bonded to benzene ring increases. The compounds having pyrimidine ring or dioxane ring at the core part like the compounds of the general formula (1-12), (1-13), (1-14), (1-16), (1-18), or (1-20) have large xcex94xcex5, the compounds having cyclohexene ring like the compounds of the general formula (1-11) are excellent in miscibility at low temperatures, and the compounds having 2-fluorocyclohexene ring like the compounds of the general formula (1-17) have a wide temperature range of nematic phase.
Among the compounds described above, the compounds of one of the general formulas (1-21) to (1-32) have large xcex94n; xcex94xcex5 of the compounds becomes larger as the number of fluorine atom bonded to benzene ring increases; the compounds having pyrimidine ring or dioxane ring have large xcex94xcex5; the compounds having cyclohexene ring are excellent in miscibility at low temperatures; and the compounds having 2-fluorocyclohexene ring exhibit nematic phase at a wide temperature range.
As described above, any of the compounds of the present invention have a halogen atom, halogenated alkyl group, or halogenated alkenyl group bonded to the carbon atom of carbonyl in at least one ester group, and have such excellent features that the compounds of the present invention have larger xcex94xcex5 value and lower viscosity while having the same extent of xcex94n compared with conventional compounds having small xcex94n, and that the compounds of the present invention have higher clearing point and low viscosity while having the same extent of xcex94n compared with conventional compounds having medium or large xcex94n.
In the compounds of the present invention, each atom which constitutes the compounds may be replaced by its isotope, and characteristics of the compounds are not changed by such replacement.
While the compounds of the present invention are suitable as a component of liquid crystal compositions particularly for TFT mode, the compounds are useful as a component of the liquid crystal compositions even for other uses, for example, for TN mode, guest-host mode, polymer dispersion mode, dynamic scattering mode, STN mode, IPS mode, OCB mode,or R-OCB mode, and further can be used as a compound for forming ferroelectric liquid crystal compositions or antiferroelectric liquid crystal compositions.
Liquid crystal compositions provided according to the present invention comprise, as a first component, at least one liquid crystalline compound expressed by the general formula (1).
The content of the compound expressed by the general formula (1) in the liquid crystal compositions of the present invention is necessary to be 0.1 to 99.9% by weight based on the amount of liquid crystal composition to develop excellent characteristics, and the content is preferably 1 to 50% by weight and more desirably 3 to 20% by weight.
While the liquid crystal compositions of the present invention may comprise only the first component described above, the compositions in which at least one compound selected from the group consisting of the compounds expressed by one of the general formulas (2), (3), and (4) (hereinafter referred to as second component A) described above and/or at least one compound selected from the group consisting of the compounds expressed by the general formula (5) or (6) (hereinafter referred to as second component B) described above is added as a second component, and at least one compound selected from the group consisting of the compounds expressed by one of the general formulas (7), (8), and (9) is further added as a third component, in addition to the first component are preferable. Still further, an optically active compound and known compounds for the purpose of adjusting threshold voltage, temperature range of liquid crystal phase, xcex94xcex5, xcex94n, viscosity, and the like can be added as other component.
Among the second component A described above, the compounds expressed by one of the general formulas (2-1) to (2-9) can be mentioned as preferable examples of the compounds expressed by the general formula (2), the compounds expressed by one of the general formulas (3-1) to (3-69) as preferable examples of the compounds expressed by the general formula (3), and the compounds expressed by one of the general formulas (4-1) to (4-24) as preferable examples of the compounds expressed by the general formula (4), respectively. 
wherein R3 and Y3 have the same meaning as described above.
The compounds expressed by one of these general formulas (2) to (4) have positive xcex94xcex5, are excellent in thermal stability and chemical stability, and are useful when liquid crystal compositions for TFT mode of which high reliability such that voltage holding ratio is high (specific resistance is large) is required are produced.
When liquid crystal compositions for TFT mode are produced, the content of the compound expressed by one of the general formulas (2) to (4) is suitably in the range of 1 to 99% by weight based on the total amount of liquid crystal composition, and the content is preferably 10 to 97% by weight and more desirably 40 to 95% by weight. At that time, the compounds expressed by one of the general formulas (7) to (9) may be used together.
While the compounds expressed by one of the general formulas (2) to (4) can be used even when liquid crystal compositions for STN mode or TN mode are produced, their content at that time is preferably less than 50% by weight based on the total amount of liquid crystal composition since the effect of the compounds of lowering threshold voltage of liquid crystal compositions is small.
Among the second component B, the compounds expressed by one of the general formulas (5-1) to (5-40) can be mentioned as preferable examples of the compounds expressed by the general formula (5), and the compounds expressed by one of the general formulas (6-1) to (6-3) as preferable examples of the general formula (6), respectively. 
wherein R4, R5, and Y4 have the same meaning as described above.
These compounds expressed by the general formula (5) or (6) have positive and large xcex94xcex5 value, and are used particularly for the purpose of lowering threshold voltage of liquid crystal compositions. Also, they are used for the purpose of improving the steepness of threshold characteristics of liquid crystal compositions for STN mode or TN mode including for the purpose of adjusting xcex94N and widening nematic range such as raising clearing point of liquid crystal compositions, and are useful compounds when liquid crystal compositions particularly for STN mode or TN mode are produced.
The compounds can lower threshold voltage of liquid crystal compositions according as their content is increased, but, on the other hand, they bring about increase of viscosity. Therefore, it becomes more advantageous for driving the devices at a low voltage as their content increases, so far as viscosity of liquid crystal compositions satisfies required characteristics.
From such circumstances, when liquid crystal compositions for STN mode or TN mode are produced, the content of the compounds of the general formula (5) or (6) is suitably in the range of 0.1 to 99.9% by weight, preferably 10 to 97% by weight, and more desirably 40 to 95% by weight based on the total amount of liquid crystal composition.
Among the third component described above, the compounds expressed by one of the general formulas (7-1) to (7-11) can be mentioned as preferable examples of the compounds expressed by the general formula (7), the compounds expressed by one of the general formulas (8-1) to (8-18) as preferable examples of the general formula (8), and the compounds expressed by one of the general formulas (9-1) to (9-6) as preferable examples of the general formula (9), respectively. 
wherein R6 and R7 have the same meaning as described above.
The compounds expressed by one of the general formulas (7) to (9) have a small absolute xcex94xcex5 value close to zero. Among them, the compounds expressed by the general formula (7) are used principally for the purpose of adjusting viscosity or adjusting xcex94n of liquid crystal compositions, the compounds expressed by the general formula (8) or (9) are used for the purpose of widening nematic range such as raising clearing point, and adjusting xcex94n of liquid crystal compositions.
These compounds raise threshold voltage of liquid crystal compositions according as their content is increased, but, on the other hand, they reduce viscosity. Thus, their content is desirably high so far as threshold voltage of liquid crystal compositions satisfies required characteristics.
From such circumstances, when liquid crystal compositions for TFT mode are produced, the content of the compounds expressed by one of the general formulas (7) to (9) is suitably less than 40% by weight, and preferably less than 35% by weight based on the total amount of liquid crystal composition. On the other hand, when liquid crystal compositions for STN mode or TN mode are produced, the content of the compounds expressed by one of the general formulas (7) to (9) is suitably less than 70% by weight and preferably less than 60% by weight based on the total amount of liquid crystal composition.
Excepting such a specific case as liquid crystal compositions for OCB (Optically Compensated Birefringence) mode, an optically active compound is generally added to the liquid crystal compositions for the purpose of inducing helical structure of liquid crystal compositions to adjust required twist angle and to prevent reverse twist. Optical active compounds added to the liquid crystal compositions of the present invention are not specifically restricted so far as the purposes described above are achieved. While they can be selected from a wide range of known optically active compounds, the optically active compounds expressed by one of the following formulas (Op-1) to (Op-8) can preferably be mentioned. 
By adding one of these optically active compounds, the pitch of the twist of liquid crystal compositions can be adjusted. Pitch of the twist is preferably adjusted in the range of 40 to 200 xcexcm in the case of liquid crystal compositions for TFT mode or TN mode, preferably adjusted in the range of 6 to 20 xcexcm in the case of liquid crystal compositions for STN mode, and preferably adjusted in the range of 1.5 to 4 xcexcm in the case of liquid crystal compositions for bistable TN mode.
At this time, two or more kind of optically active compounds may be added for the purpose of adjusting the dependency of the pitch length on temperature.
The liquid crystal compositions provided according to the present invention can be produced by methods which are conventional by themselves, for instance, by a method in which various components are dissolved in one another at a high temperature.
Further, the liquid crystal compositions of the present invention can be used as ones for guest-host (GH) mode by adding a dichroic dye such as merocyanine type, styryl type, azo type, azomethine type, azoxy type, quinophthalone type, anthraquinone type, and tetrazine type thereto. Furthermore, the liquid crystal compositions can be used as NCAP which is prepared by the microencapsulation of a nematic liquid crystal, or as liquid crystal compositions for polymer dispersed liquid crystal display devices (PDLCD) represented by polymer net work liquid crystal display devices (PNLCD) prepared by forming a polymer of three-dimensional reticulated structure in a liquid crystal. Still further, the liquid crystal compositions of the present invention can be used as ones for electrically controlled birefringence (ECB) mode or dynamic scattering (DS) mode.
As examples of the nematic liquid crystal compositions of the present invention, the following composition examples 1 through 41 can be shown.
Each of the compounds in the following composition examples is designated by making groups shown in each of the columns of left side terminal group, bonding group, ring structure, and right side terminal group correspond to symbols shown in each of their columns according to the definitions shown in Table 1 described below. Particularly, positions of deuteriums on cyclohexane ring are indicated by the numerals added at right side shoulder of L in Table 2, and designated like examples shown in the Table together with symbol D indicating the deuterium.
Compounds shown in the compositions examples and those in the examples described below to both of which the same Compound No. is added means the same compound, and the content of compounds is shown in % by weight unless otherwise specified.
Characteristic data in Composition Examples are shown by TNI (nematic-isotropic liquid phase transition temperature or clearing point), xcex7 (viscosity: determined at 20.0xc2x0 C.), xcex94n (optical anisotropy value: determined at 25.0xc2x0 C.), xcex94xcex5 (dielectric anisotropy value: determined at 25.0xc2x0 C.), and Vth (threshold voltage: determined at 25.0xc2x0 C.).
Characteristic values of this composition were as follows:
TNI=70.4 (xc2x0 C.)
xcex7=20.8 (mPaxc2x7s)
xcex94n=0.079
xcex94xcex5=5.0
Vth=2.31 (V)
Characteristic values of this composition were as follows:
TNI=67.1 (xc2x0 C.)
xcex7=22.5 (mPaxc2x7s)
xcex94n=0.104
xcex94xcex5=5.1
Vth=2.16 (V)
Characteristic values of this composition were as follows:
TNI=77.3 (xc2x0 C.)
xcex7=18.5 (mPaxc2x7s)
xcex94n=0.110
xcex94xcex5=4.8
Vth=2.39 (V)
Characteristic values of this composition were as follows:
TNI=96.5 (xc2x0 C.)
xcex7=14.2 (mPaxc2x7s)
xcex94n=0.094
xcex94xcex5=5.1
Vth=2.18 (V)
Pitch of the liquid crystal composition prepared by adding 0.3 part by weight of the optically active compound expressed by the formula (Op-8) described above to 100 parts by weight of the composition 4 described above was 76.8 xcexcm.
Characteristic values of this composition were as follows:
TNI=88.8 (xc2x0 C.)
xcex7=15.3 (mPaxc2x7s)
xcex94n=0.163
xcex94xcex5=7.5
Vth=1.99 (V)
Pitch of the liquid crystal composition prepared by adding 0.8 part by weight of the optically active compound expressed by the formula (Op-4) described above to 100 parts by weight of the composition 5 described above was 11.7 xcexcm.
Characteristic values of this composition were as follows:
TNI=93.4 (xc2x0 C.)
xcex7=32.9 (mPaxc2x7s)
xcex94n=0.114
xcex94xcex5=8.9
Vth=1.81 (V)
Pitch of the liquid crystal composition prepared by adding 0.25 part by weight of the optically active compound expressed by the formula (Op-5) described above to 100 parts by weight of the composition 6 described above was 61.1 xcexcm.
Characteristic values of this composition were as follows:
TNI=88.0 (xc2x0 C.)
xcex7=17.2 (mPaxc2x7s)
xcex94n=0.094
xcex94xcex5=5.1
Vth=2.11 (V)
Characteristic values of this composition were as follows:
TNI=90.5 (xc2x0 C.)
xcex7=32.1 (mPaxc2x7s)
xcex94n=0.120
xcex94xcex5=6.2
Vth=2.03 (V)
Characteristic values of this composition were as follows:
TNI=79.2 (xc2x0 C.)
xcex7=18.2 (mPaxc2x7s)
xcex94n=0.139
xcex94xcex5=8.0
Vth=1.96 (V)
Characteristic values of this composition were as follows:
TNI=74.4 (xc2x0 C.)
xcex7=68.8 (mPaxc2x7s)
xcex94n=0.121
xcex94xcex5=24.8
Vth=1.20 (V)
Characteristic values of this composition were as follows:
TNI=76.7 (xc2x0 C.)
xcex7=15.1 (mPaxc2x7s)
xcex94n=0.130
xcex94xcex5=7.4
Vth=1.88 (V)
Characteristic values of this composition were as follows:
TNI=71.1 (xc2x0 C.)
xcex7=31.8 (mPaxc2x7s)
xcex94n=0.083
xcex94xcex5=13.0
Vth=1.39 (V)
Characteristic values of this composition were as follows:
TNI=91.9 (xc2x0 C.)
xcex7=15.2 (mPaxc2x7s)
xcex94n=0.152
xcex94xcex5=7.8
Vth=2.02 (V)
Pitch of the liquid crystal composition prepared by adding 0.8 part by weight of the optically active compound expressed by the formula (Op-4) described above to 100 parts by weight of the composition 21 described above was 11.3 xcexcm.
Characteristic values of this composition were as follows:
TNI=90.6 (xc2x0 C.)
xcex7=88.3 (mPaxc2x7s)
xcex94n=0.147
xcex94xcex5=31.7
Vth=0.83 (V)
Characteristic values of this composition were as follows:
TNI=68.6 (xc2x0 C.)
xcex7=40.3 (mPaxc2x7s)
xcex94n=0.119
xcex94xcex5=12.3
Vth=1.26 (V)
Characteristic values of this composition were as follows:
TNI=75.1 (xc2x0 C.)
xcex7=19.3 (mPaxc2x7s)
xcex94n=0.135
xcex94xcex5=8.8
Vth=1.66 (V)
Characteristic values of this composition were as follows:
TNI=72.6 (xc2x0 C.)
xcex7=36.8 (mPaxc2x7s)
xcex94n=0.110
xcex94xcex5=24.4
Vth=0.96 (V)
Characteristic values of this composition were as follows:
TNI=96.7 (xc2x0 C.)
xcex7=19.3 (mPaxc2x7s)
xcex94n=0.096
xcex94xcex5=5.2
Vth=2.47 (V)
Characteristic values of this composition were as follows:
TNI=77.4 (xc2x0 C.)
xcex7=13.5 (mPaxc2x7s)
xcex94n=0.124
Vth =1.96 (V)
Characteristic values of this composition were as follows:
TNI=71.7 (xc2x0 C.)
xcex7=23.6 (mPaxc2x7s)
xcex94n=0.075
xcex94xcex5=5.9
Vth=2.08 (V)
Pitch of the liquid crystal composition prepared by adding 0.3 part by weight of the optically active compound expressed by the formula (Op-8) described above to 100 parts by weight of the composition 28 described above was 72.8 xcexcm.
Characteristic values of this composition were as follows:
TNI=70.6 (xc2x0 C.)
xcex7=24.2 (mPaxc2x7s)
xcex94n=0.099
xcex94xcex5=6.4
Vth=1.91 (V)
Characteristic values of this composition were as follows:
TNI=77.8 (xc2x0 C.)
xcex7=19.8 (mPaxc2x7s)
xcex94n=0.114
xcex94xcex5=5.3
Vth=2.21 (V)
Characteristic values of this composition were as follows:
TNI=94.7 (xc2x0 C.)
xcex7=34.5 (mPaxc2x7s)
xcex94n=0.113
xcex94xcex5=9.2
Vth=1.73 (V)
Pitch of the liquid crystal composition prepared by adding 0.25 part by weight of the optically active compound expressed by the formula (Op-5) described above to 100 parts by weight of the composition 31 described above was 62.3 xcexcm.
Characteristic values of this composition were as follows:
TNI=78.7 (xc2x0 C.)
xcex7=15.3 (mPaxc2x7s)
xcex94n=0.088
xcex94xcex5=5.0
Vth=2.31 (V)
Characteristic values of this composition were as follows:
TNI=96.2 (xc2x0 C.)
xcex7=34.3 (mPaxc2x7s)
xcex94n=0.127
xcex94xcex5=7.4
Vth=1.90 (V)
Characteristic values of this composition were as follows:
TNI=82.9 (xc2x0 C.)
xcex7=19.3 (mPaxc2x7s)
xcex94n=0.088
xcex94xcex5=3.9
Vth=2.41 (V)
Characteristic values of this composition were as follows:
TNI=70.1 (xc2x0 C.)
xcex7=25.9 (mPaxc2x7s)
xcex94n=0.081
xcex94xcex5=8.9
Vth=1.52 (V)
Characteristic values of this composition were as follows:
TNI=69.3 (xc2x0 C.)
xcex7=25.7 (mPaxc2x7s)
xcex94n=0.089
xcex94xcex5=8.6
Vth=1.70 (V)
Characteristic values of this composition were as follows:
TNI=72.8 (xc2x0 C.)
xcex7=17.6 (mPaxc2x7s)
xcex94n=0.082
xcex94xcex5=5.0
Vth=2.08 (V)
The compounds of the present invention expressed by the general formula (1) can readily be produced by the methods described in known literatures, for example, xe2x80x9cJikken Kagaku Kouza (Course of Chemical Experiment)xe2x80x9d fourth edition (Maruzen) and J. Org. Chem., 62, 726 (1997).
Among the compounds expressed by the general formula (1), compound (18) wherein both Y1 and Y2 are oxygen atoms can preferably be produced by reacting a compound having a hydroxyl group R1xe2x80x94(A1xe2x80x94Za)n1xe2x80x94(A2xe2x80x94Zb)n2xe2x80x94(A3xe2x80x94Zc)n3xe2x80x94A4xe2x80x94Zdxe2x80x94A5xe2x80x94Zexe2x80x94OH (16) with acid anhydride (17) in the presence of a base such as pyridine. Compound (22) can be produced even by other methods, for instance, by synthesizing compound (21) from benzotriazole (19) and acid anhydride (20) by using the method described in J. Org. Chem., 62, 726 (1997), and then reacting the compound (21) with compound (16), or by synthesizing compound (24) from 2-pyridinol (23) and acid anhydride (20) by using the method described in Bull. Chem. Soc. Jpn., 63, 2252, and then reacting the compound (24) with compound (16). 
Compound (30) expressed by the general formula (1) wherein Ze is single bond and A5 is 1-cyclohexene-1,4-diyl can be produced by synthesizing compound (27) from trifluoroacetic acid (25) and compound (26) by using the method described in J. Org. Chem., 44, 313 (1979), and then reacting the compound (27) with a cyclohexanone (28) and compound (29). 
wherein R1, R2, A1 to A5, Za to Ze, n1 to n3, Y1, and Y2 have the same meaning as described above.